Microwave oven having preferred modes

ABSTRACT

A counter-top microwave oven comprises an upstanding box-like metal wall structure defining a heating cavity, a microwave generator including a voltage quadrupler power supply, and transmission means including a probe antenna for radiating the microwaves into the heating cavity, the cavity being so dimensioned and the antenna being so positioned as to excite in said cavity preferred electromagnetic field modes for establishing preferred field patterns to achieve uniform heating of a variety of foods. A first embodiment of the heating cavity is dimensioned to produce two complementary modes of which establish an undistorted field pattern providing uniform heating over a distance greater than a wavelength of the source microwave energy, tuning means being provided to tune undesired modes of resonance. A second embodiment of the oven is dimensioned to produce a single electromagnetic field mode producing a field pattern which provides uniform heating over a distance less than a wavelength of the source microwave energy. Several forms of each embodiment of the invention are disclosed.

United States Patent [191 Staats et al.

[ Dec. 17, 1974 1' MICROWAVE OVEN HAVING PREFERRED MODES [75] Inventors: James E. Staats; Louis H.

Fitzmayer, both of Louisville, Ky.

[73] Assignee: General Electric Company,

Louisville, Ky.

Jan. 4, 1974 [22] Filed:

[21] Appl. No.: 430,730

Primary Examiner-Bruce A. Reynolds Attorney, Agent, or FirmPrangley, Dithmar, Vogel, Sandler & Stotland [5 7] ABSTRACT A counter-top microwave oven comprises an upstanding box-like metal wall structure defining a heating cavity, a microwave generator including a voltage quadrupler power supply, and transmission means including a probe antenna for radiating the microwaves into the heating cavity, the cavity being so dimensioned and the antenna being so positioned as to excite in said cavity preferred electromagnetic field modes for establishing preferred field patterns to achieve uniform heating of a variety of foods. A first embodiment of the heating cavity is dimensioned to produce two complementary modes of which establish an undistorted field pattern providing uniform heating over a distance greater than a wavelength of the source microwave energy, tuning means being provided to tune undesired modes of resonance. A second embodiment of the oven is dimensioned to produce a single electromagnetic field mode producing a field pattern which provides uniform heating over a distance less than a wavelength of the source microwave energy. Several forms of each embodiment of the invention are disclosed.

21 Claims, 19 Drawing Figures 1mm l,

ill/

' MICROWAVE OVEN HAVING PREFERRED MODES BACKGROUND OF THE INVENTION The present invention relates to electronic heating apparatus of the type commonly referred to as a microwave oven, and particularly to microwave ovens which can be placed atop a table or counter and which have a heating cavity with dimensions generally comparable to a wavelength of the microwave energy used. More specifically, the present invention relates to improved microwave ovens which can be operated on standard l volt a.c. household current without using a power transformer.

Microwave cooking is a complex problem and the heating characteristicsfor microwave ovens and for various types of foods have been mathematically determined for only the simplest cases. It is recognized, however, that the ability of amicrowave oven to efficiently and uniformly cook a food load is dependent upon the electromagnetic field pattern which is established in the heating cavity. Normally, a microwave oven is adapted toopera'te at a predetermined ultrahigh frequency and, typically, aconventional microwave oven of arbitrary dimensions is found to have electromagnetic field modes near the desired wavelength. In practice, one 'of these'modes, usually a transverse electric (TE) mode is excited in the heating cavity, and this mode is characterized by both high impedance and low impedance regions in the heating cavity. Generally, the position in the heating cavity in which food is normally placed lies in'the high impedance region of the electromagnetic field pattern.

But most food itemshave a relatively low, impedance in the range of '3 to 200 ohms and, typically, the food presents a series impedance with the standing wave impedance in the heating cavity so that heating occurs in the low impedance regionof the wave pattern. Thus, when food is placed in the oven ina high impedance region or partially in high impedance region and partially in alow impedance region, the result is that the food either does not heator the heating is not uniform.

Heretofore, this problem was attacked-by using rotating mode stirrers in the heating cavity to excite a large number of'electromagnet field'modes in the hope that the combined effect of these modes would be to produce a more uniform heating pattern. Similarly, in the copending U.S. Application Ser. No. 317,206, filed Dec. 2l, 1972, and the copending US. Application Ser. No. 320,140, filed Jan. 2, 1973, both assigned to the assignee of the present invention, rotating mode excitersare utilized to excite and couple specific secondary modes to provide a more uniform time-averaged field distribution in the heating cavity. Alternatively, rotating turntable-type shelves have been provided in the heating cavity for supporting the food and moving the food through different regions of the field pattern so as to achieve a more uniform heating of the food. But all of these prior art attempts at improving the uniformity of heating in the microwave oven involve the vide an electronic heating apparatus characterized by a distortion-free electromagnetic field pattern which affords uniform heating of food without requiring a mode stirrer or turntable or the like. More particularly, it has been found that unique combinations of heating cavity dimensions and microwave antenna location will produce in the heating cavity preferred electromagnetic field modes which provide improved uniformity of heating of the food in its normal position in the heating cavity.

The present invention is also intended to provide an improved power supply for an electronic oven whereby the microwave generator can be operated on normal low voltage household current.

It is an important object of the present invention to provide an electronic heating apparatus comprising a metal enclosure including six rectangular walls arranged to form a rectangular parallelepiped defining a heating cavity for receiving therein a body to be heated, and source means for transmitting microwave energy of a predetermined wavelength through one of the walls into the heating cavity for exciting therein a predetermined electromagnetic field mode, the one wall having an electrical dimension D such that wherein n is any integer and A is the predetermined wavelength, the predetermined electromagnetic field mode having the fields thereof constant in directions normal to the one wall with power flow being parallel to the direction of the dimension D, whereby there is established in the heating cavity an electromagnetic field pattern providing uniform heating of an associated body. 7

it is another object of this invention to provide an electronic heating apparatus of the type set forth wherein the source means is electrically coupled to the heating cavity at a location along a rear. wall thereof in the upperhalf thereof for exciting in the cavity two predetermined complementary electromagnetic field modes, atleast one of the predetermined electromagnetic field modes having the fields thereof constant in a direction parallel to oneof the dimensions of the heating cavity, each of the predetermined electromagnetic field-modes having the fields thereof varying in the direction in which the fields of the other mode are constant.

In connection with the foregoing object, it is another object of this invention to provide electronic heating apparatus of the type set forth, which further includes tuning means disposed in the heating cavity and coupled to the top wall thereof for couplingelectromagnetic field modes other than the predetermined'complementary modes and tuning the other-modes off resonance.

use of moving parts which add to the complexity, the

expense and the difficulty of maintenance of the electronic oven.

Still another object of this invention is to provide electronic heating apparatus of the type set forth, which includes a voltage converter having a pair of input terminals adapted to be connected to an associated low frequency a.c. supply source having a voltage in the general range of to l25 volts and a pair of output terminals respectively connected to the input terminals of a crossed-field discharge device, the converter being characterized by the production of a d.c. output voltage across the output terminals thereof having a maximum amplitude approximately four times the peak value of the a.c. input voltage to the converter.

Further features of the invention pertain to the particular arrangement of the parts of the electronic heating apparatus whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and the method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front perspective view of an electronic heating apparatus constructed in accordance with and embodying the features of the present invention;

FIG. 2 is an enlarged horizontal cross-sectional view of the electronic eating apparatus of FIG. 1 with the top wall thereof removed, and with the top wall of the heating cavity partially broken away;

FIG. 3 is a view in vertical section of the electronic heating apparatus of FIG. 2 as viewed from the righthand side thereof with the right-hand sidewall removed;

FIG. 4 is a further enlarged fragmentary diagrammatic view of the microwave trap in the door of the electronic heating apparatus of FIG. 1;

FIG. 5 is a schematic electrical circuit diagram of the control and the power supply circuitry of the electronic heating apparatus of FIG. 1;

FIG. 6 is a diagrammatic transparent perspective view of a first form of the heating cavity of the electronic heating apparatus of FIG. 1 for producing two complementary electromagnetic field modes;

FIG. 7 is a diagrammatic top plan view of the heating cavity illustrated in FIG. 6, and showing the electric field pattern of the TE mode therein;

FIG. 8 is a diagrammatic side elevational view of the heating cavityshown in FIG. 6, illustrating the electric fields of the TE mode therein;

FIG. 9 is a diagrammatic transparent perspective view similar to FIG. 6 of a second form of the heating cavity for the electronic heating apparatus of FIG. 1; FIG. 10 is aview similar to FIG. 9 showing a third form of the heating cavity for'the electronic heating apparatus of FIG. 1; 1 i

FIG. 11 is a view similar to FIG. 9 showing a fourth form of the heating cavity for the electronic heating apparatus of FIG. 1;

, FIG. 12 is a view similar to FIG. 9 showing a fifth form of the heating cavity of the electronic heating apparatus of FIG. 1;

FIG. 13 is a view similar to FIG. 9 showing a sixth form of the heating cavity of the electronic heating apparatus of FIG.-1;

- fields thereof;

FIG. 16 is a diagrammatic transparent perspective view of a second embodiment of the heating cavity of the electronic heating apparatus of the present invention for producing a single TE mode;

FIG. 17 is a view similar to FIG. 16 showing a second form of the heating cavity of FIG. 16;

FIG. 18 is a view similar to FIG. 16 showing a third form of the heating cavity of FIG. 16; and

FIG. 19 is a fragmentary perspective view of an alternative forrn of antenna for the electronic heating apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in particular to FIGS. 1 through 5 of the drawings, there is illustrated a microwave oven, generally designated by the numeral 20, constructed in accordance with and embodying the features of the present invention. The microwave oven 20 is adapted for placement on top of a table or counter and is housed in a cabinet which includes an upstanding front panel 21, a rear wall 22, a top wall 23, a bottom wall 24 and a pair of opposed sidewalls 26 and 27. Mounted within the oven cabinet alongside the front panel 21 is a heating enclosure, generally designated by the numeral 30, including a top wall 31, a bottom wall 32, a rear wall 33 and a pair of opposed sidewalls 34 and 36,

. the front of the heating enclosure 30 being closed by a door 35 which, in the closed position thereof, forms the front wall of the heating enclosure 30, which enclosure is generally in the form of a rectangmlar parallelepiped.

the top wall 31 is a broil heating element 37. Mounted in the sidewall 36 of the heating enclosure 30 projecting inwardly of the heating cavity is a thermostat 38 which is electrically connected to a control knob on the front panel 21., The oven door 35 is provided around the periphery thereof with a groove or cavity 39 having an effective electrical length equal to one-fourth of the wavelength A" of the microwave energy supplied to the heating enclosure 30, thereby serving as a trap to prevent the escape of microwave energy from the heating enclosure 30 in use.

. Staats on Dec. 29, 1970, and assigned to the assignee of the present invention. The magnetron 40 is adapted to produce microwave energy having a frequency of approximately 915 MHz. at the output terminals thereof when a dc. voltage of approximately 560 volts is applied to the input terminals thereof. Coupled to the output terminals of the magnetron 40 is a coupler and filter device 43, the input terminals of the magnetron 40 being provided with a filter 46, the filters 43 and 46 filtering out the higher harmonics of the fundamental frequency of the microwave output of the magnetron 40 and also serving to isolate the dc. power supply from the microwave energy. The coupler and filter device 43 is coupled to a coaxial transmission line 41 by means of a capacitive coupling 42, the transmission line 41 in turn being coupled to a probe antenna 44 which projects through an opening in the rear wall 33 of the heating enclosure 30 into the heating cavity for radiating microwave energy thereinto. A transmission line 41 and the capacitive coupling 42 may be of the type illustrated in the U.S PatQNo. 3,626,135 issued to Louis H. Fitzmayer on Dec. 7, 1971, and assigned to the assignee ofv the present invention. Mounted on the top wall 31 of the heating enclosure 30 and projecting downwardly into the heating cavity is a metallic tuning member 45 for a purpose to be described more fully hereinafter.

Mounted in the machinery compartment 25 immediately beneath the magnetron 40 is a blower 47 adapted tral conductor N of a standard 220 240 volt 60 Hz. sin

gle-phase three-wire Edison supply network, the voltage between L, and N being in the range I -120 volts.

Power line L, is connected, through a fuse -51 to an interlock switch 52 which is in the door 35, the switch 52 being closed only when the door 35 is closed to insure that the magnetron 40 cannot be energized unless the door 35 is securely closed and locked, thereby protecting against the inadvertent escape of microwave enis a series combination of two input capacitors 75 and 76, the junction therebetween being connected to neutral through the parallel combination of a thermistor 66, a resistor 67 and normally-open contacts 82 of the relay coil 60.

Connected across the rectifiers 71, 72,73 and 74 is the series combination of two output capacitors 77 and 78, the junction therebetween being connected to the movable contact 55C of the selector switch 55, the HI fixed contact of which is designated by the numeral 80 and also forms a fixed pole of normally-open contacts 79 of the-relay 60, the movable arm of the contacts 79 being connected to the junction between the rectifiers 71 and 72. The output of the voltage quadrupler 70 is taken across the output capacitors 77 and 78, the junction between the capacitor 77 and the rectifier 73 being connected through magnet coils 68 and 69 to the anode of the magnetron 40, while the junction between the capacitor 78 and the rectifier 74 is connected to the junction between the relay coil 60 and the capacitor Referring now also to FIGS. 6 through 8 of the drawings, the operation of the microwave oven 20 will be described. In practice, a food load F is placed in the heating enclosure in the position illustrated in FIG. 6. The door is then closed, thereby closing the interlock switch 52 and permitting the electronic oven to be operated. At this point the control and power supply ergy from the heating enclosure 30. The interlock coil 54 of a voltage quadrupler circuit 70, and is also connected by a conductor-'56 to the movable contact 553 of the selector switch '55, the LO position fixed contact of which is connected to one terminal of the primary winding of a transformer 61, the otherterminal of which winding is connected to neutral. The secondary-coil of the transformer 61 is connected across the heater element of the magnetron 40.

7 Connected in parallel with the contact 55B of the selector switch 55 area resistor 58 and the normallyclosed contacts 59 of a relay coil 60. Oneterminal of the relay coil 60 is connected to the secondary of the transformer 61 and the other terminal thereof is connected through a capacitor 62 to the anode of the mag-. netron 40.

The L0 fixed contact of the selector switch movable contact A is connected to the conductor 53 and is also connected by a conductor 63 to neutral through the parallel combination of a panel lamp 64 and a motor 65 for the blower 47. Connected in series between the BROIL contact of the selector switch movable contact 55A in neutral are the BROIL heating element 37 and the thermostat 38. The choke coil 54 is connected to the junction between two seriesconnected rectifiers 71 and 72, the other terminals of which are respectively connected in series to rectifiers 73 and 74. Connected across the rectifiers 71 and 72 assembly 50 will be in the condition illustrated in FIG. 5, with the selector switch 55 in the OFF position. If it is desired to operate the microwave oven 20 on high power, the selector switch 55 is moved to the HI position. This results in energization of the panel lamp 64 and the blower motor 65 through the conductor 63, activation of the input voltage doubler portion of the voltage quadrupler through the conductor 53 and energization of the transformer '61 through the normallyclosed contacts 59 of the relay 60. The contact 55C of the selector switch 55 is moved to the fixed contact pole 80.

Thus, at start, a relatively low voltage from the input voltage doubler portion of the voltage quadrupler 70 is applied to the anode of the magnetron 40, while full heater power is maintained through the'relay contacts 59. Initially, when the magnetron 40 conducts,the voltage across the resistor 67 heats the thermistor 66 causing a decrease in its resistance and, thereby, an increase in magnetron voltage. When the magnetron current reaches a predetermined value, the relay 60 will be actuated, thereby closing its normally-open contacts 79 and 82 and opening its normally-closed contacts 59.

Preferably, the relay contacts 82 close first, thereby shorting out the thermistor 66 and the resistor 67, resulting in an increase in voltage and current thereby to provide positive pull in of the relay 60. Closure. of the relay contacts 79 activates the voltage quadrupler 70 I through the selector switch contact 55C, thereby providing approximately 560 volts dc. to the anode of the magnetron 40. Opening of the relay contacts-59 places the resistor 58 in series with the primary of the transformer 61 thereby reducing the heater current of the magnetron 40. The magnetron 40 is then operating on full high power and will continue in that mode of operation until the position of the selector switch 55 is changed.

When it is desired to operate the electronic oven 20 on low power, the selector switch 55 is moved to the LO position, in which position the input voltage doubler portion of the voltage quadrupler 70, the panel lamp 64 and the blower motor 65 are all energized in the same manner as was described above with respect to the high power mode of operation. The primary of the transformer 61 is energized through the conductor 56 and the closed contacts 55B of the selector switch 55, as well as through the normally-closed contacts 59 of the relay 60. The startup of the magnetron 40 is the same as was described above in connection with the high power mode of operation, except that when the relay 60 is actuated the closure of the relay contacts 79 does not actuate the voltage quadrupler 70 because the selector switch contact SSC is out of contact with the pole 80, and the opening of the relay contacts 59 does not reduce the heater current, because the resistor 58 remains shorted out by the selector switch contact 55B.

When it is desired to broil or brown the food in the heating enclosure 30, the selector switch 55 is moved to the BROIL position, thereby de-energizing the magnetron 40 and energizing the broil heating element 37 through the thermostat 38.

It is a significant feature of the present invention that the voltage quadrupler 70 can be switched between a low power configuration which utilizes only the input itance of the output capacitors 77 and 78.

The microwave energy at the output of the magnetron is transmitted through the coupler and filter member 43, the capacitive coupling 42 andthe transmission line 41 to the antenna 44 which radiates the microwaves into-the heating enclosure 30 for establishing therein a predetermined electromagnetic field pattern. It is an importantfeature of the present invention that the antenna 44 is so positioned, andthe heating enclosure 30 is so dimensioned that two predetermined complementary electromagnetic field modes are established in the heating enclosure 30. The predetermined modes complement each other in that each mode has fields which vary in the directions in which the fields of the other mode are constant, thereby producing a composite electromagnetic field patterh which varies in all three dimensions of the heating enclosure 30. Those dimensions are such that only selected complementary modes are excited in the heating cavity, which modes will produce an undistorted field pattern which provides a uniform heating pattern greater than A, where A is the wavelength of the microwave energy generated at the magnetron 40. Furthermore, the variation of the electromagnetic fields in all three dimensions of the heating cavity facilitates the cooking of foods of various shapes and sizes.

In the 'form of the heating enclosure 30 illustrated in FIG. 6, the dimensions of the heating cavity are in accordance with the relationships and wherein x corresponds to the width of the heating cavity, y corresponds to the height of the heating cavity and z corresponds to the depth of the heating cavity. From these relationships it follows that:

It has been found that when the heating enclosure 30 has dimensions according to these relationships, such thatx= 1.2 X, y =O.633 A and z 1.1 A, and when the antenna 44 is positioned as illustrated in FIG. 6 substantiallymidway between the side walls 34 and 36 and approximately y/4 from the top wall 31, there are produced in the heating cavity the TE and TE electromagnetic field modes, wherein the numeral subscripts represent, respectively, the field variations in the x, y and z directions, and the letter subscript indicates the direction of power flow.

There is illustrated in FIG. 15 a side view of the horizontal and vertical fields at the antenna 44 in the heating enclosure 30 of FIG. 6. A top view of the electric fields of the TE mode established in the cavity of FIG. 6 is illustrated in FIG. 7 and is designated by the numeral 85, while a side view of the electric fields of the TE mode established in the heating enclosure of FIG. 6 is shown in FIG. 8 and is designated by the numeral 86. In practice it has been found that the TE mode provides heating of the outer portions of the food in the heating cavity, while the T TE mode provides heating of the inner portions of the food. In order to enhance the uniform heating pattern of the complementary field modes established in the heating enclosure30, the tuning element 45 is'provided sub stantially centrally of the top wall 31 projecting downwardly into the heating cavity. The tuning element 45 is so' designed and positioned that it does not couple the desired TE,,,,,,, and TE modes but it does couple undesired TEand TM (transverse magnetic) modes and tunes these undesired modes off resonance. At this point it should be noted that the dimensions of the heating cavity referred to herein are effective electrical dimensions and that, by reason of the effect of the tuning element 45, these electrical dimensionsrnay differ from the mechanical dimensions of the heating enclosure.

A number of different forms of the invention embodying different antenna placements and different heating cavity dimensions have been discovered which will provide two complementary transverse electric modes which have the desired uniform heating pattern. Several of these different forms of the invention are illustrated in FIGS. 9 through 14. In FIG. 9 there is illustrated a heating enclosure which is substantially in the form of a cube with dimensions M 2 the antenna 44 being disposed along the rear wall midway between the side walls and approximately y/4 below the top wall. The heating enclosure 100 has produced therein the TE and the TE modes, whereby both horix/4from the left sidewall and approximately y/4 from the top wall. This arrangement serves to produce in the heating enclosure 1 the TE and the TE complementary electromagnetic field modes.

Similarly, in FIG. 11 there is disclosed a heating enclosure l-which is cubical in shapehaving dimensions V2 A, with the antenna 44 being disposed along the rear wall and spaced approximately x/ 4 from the left sidewall and approximately y/4 from the top wall. This configuration produces in the heatingenclosure 120 the TE and the TE complementary electromagnetic field modes.

In FIG. 12 there is illustrated a heating enclosure 130 wherein x =3 Vim, y A/ V2 and z \/2 A, with the antenna 44-beingdisposed along the rear wall substantially midway between the sidewalls and spaced approximately y/4fromthe top wall. This configuration produces in the heating enclosure 130 the TE and TE complementary electromagnetic field modes.

- In FIG. 13 there is disclosed a heating enclosure 140 wherein x 1.61 )t, y 0.767 )t and z 1.23 A, with the antenna 44.beingdisposed along the rear wall substantially midway between the sidewalls and spaced approximately y/4 from the top wall. This configuration serves to produce in the heating enclosure 140 the TIE- and TE complementary electromagnetic field modes.

In FIG. 14 there is disclosed a heating enclosure 150, which is similar to theheating enclosure illustrated in FIG. 6, and produces therein the same electromagnetic field modes'as are produced in the heating enclosure 30. However, in the heating enclosure 150, the x and 2 dimensions are the same, both being-equal to 1.12 A, and'y =0.645,)\.

When the heating enclosure is considered as a rectangular wave guide terminated by shorts on both ends, the guide wavelength A, is measured in the direction of power flow and determines'the characteristic impedance of Z, of the heating enclosure according to the relationship I D a 1.1 n M2 0.55" x.

Referring now to FIG/16 of the drawings, there isillustrated another embodiment 200 of the heating enclosure for use in the electronic oven 20 of the present invention. The heating enclosure 200 is similar to the heating enclosure 30 described above, and includes a top wall 201, a bottom wall 202, a rear wall 203 and two opposed vside walls 204 and 206, the front wall 205 being provided by the oven door in a closed position thereof. A probe antenna 207, essentially identical to the probe antenna 44 described above, is coupled to the source .of. microwave energy and projects into the heating cavity defined by the heating enclosure 200 for establishing electromagnetic field patterns therein.

The heating enclosure 200 is so designed and the antenna 207 is so positioned therein as to excite a single preferred electromagnetic field mode having fields varying in two dimensions of the heating cavity and providinga heating pattern such that when food is placed in the normal position in the electronic oven it 7 will be in the low impedance region of the standing wave, the field 'pattem providing uniform heating over a distance less than A. As described above, the dimension D of the heating enclosure in the direction of power flow should be D 20.55nl1 in order that the characteristic impedance of the heating enclosure 200 shall have a suitably low value.

The dimensions of the heating enclosure 200 are determined by the relationship Alternative forms of the heating enclosure 200 illustrated-in FIG. 16 are shown in FIGS. 17 and 18. In FIG. 17 there is illustrated the heating enclosure 220 wherein x 1.2 A and z 1.1 A with y being any suitable value, the dimensions of the heating cavity being determined in accordance with the formula A= 2/ (1/x) (2/z) The antenna 207 is disposed in the top wall 201 substantially midway between the sidewalls 204 and 206 and spaced from the rear wall approximately z/4. In this configuration, the heating enclosure 200 produces therein the TE electromagnetic field mode.

In FIG. l8 there is disclosed a heating enclosure 230 wherein x 1.1 A and y 1.2 )1, with 2 being any suitable value, the heating cavity dimensions being determined according to the formula The antenna 207 is disposed in the rear wall 203 and is spaced approximately x/4 from the sidewall 206 and approximately y/4 from the top wall 201. ln this configuration, the heating enclosure 230 produces therein the TE electromagnetic field mode. It will be noted that in heating enclosure 200, 220, and 230, the direction of power fiow is parallel to the wall through which the antenna 207 enters-which wall has an electrical width D and length E, where D has a value according to the relationship defined above.

There is additionally provided in the heating enclosure 230 a metallic shelf or rack 240 comprising a grillwork including a plurality of parallel rods 245 disposed perpendicular to the electric field, the rack 240 being adapted to be positioned at various vertical levels in the lower region of the heating enclosure 230. By use of the vertically adjustable shelf 240, different impedance antenna which is approximately two to three inches in length. In FIG. 19 there is disclosed an alternative embodiment 250 of the probe antenna, whereby the length of the antenna may be substantially lessened. The antenna 250 includes a probe or stub member 252 coupled to the inner conductor of the coaxial transmission line 41, the probe 252 being terminated at its inner distal end by a circular metallic disc 255 which is secured to the probe 252 coaxially therewith. By use of this form of disc antenna, it has been found that the preferred electromagnetic field mode may be excited in the heating enclosure 200 wherein the antenna probe 252 is approximately 1 inchin length and three-eights inch in diameter, with the disc 255 having a diameter of 2 inches. These antenna dimensions may be varied if necessary to adjust coupling and tuning of the electromagnetic, field modes in the heating cavity.

In a constructional example of the control and power supply assembly 50 of the electronic oven 20, the input capacitors 75 and 76 of the voltage quadrupler 70 each have a capacitance of 500 microfarads, while each of the output capacitors 77 and 78 has a capacitance of 250 microfarads. The capacitor 62 has a capacitance of 60 microfarads, while the start resistor 67 has a resistance of thirty ohms. In use, the electronic oven has operated satisfactorily in tests with the modes illustrated in FIG. 6, a cup of water placed anywhere in the x-z plane of FIG. 6 having been heated with power equal to or greater than unity power of the magnetron 40. It has been found that small or large loads are equally well heated.

Similarly, in tests of the heating enclosure 220 of F IG. 17, the heating characteristics have been found to be good for a wide range of food loads. For example, a cup of water placed in any of 20 different positions in the heating cavity was heated with a maximum power of 750 watts and a minimum power of 600 watts.

From the foregoing, it can be seen that there has been provided a novel electronic oven which affords.

uniform heating of a wide variety of food loads, without requiring any moving parts in the heating cavity.

More particularly, there has been provided an electronic heating apparatus wherein microwave energy is radiated into the heating cavity by aprobe antenna, the placement of the probe antenna and the dimensions of the heating cavity being such as to provide excitation of two predetermined complementary electromagnetic field modes in the heating cavity which have fields varying in all three dimensions and provide a uniform heating pattern greater than one wavelength of the microwave energy.

There has also been provided a novel and improved magnetron power supply in an electronic oven of the character described, which power supply permits the magnetron to operate from normal low voltage household a.c. electric current.

More particularly, there has been provided a novel voltage quadrupler power supply for a magnetron, characterized by an improved power factor and means for switching the power supply between a low voltage mode and a high voltagemode of operation.

There has also been provided another embodiment of heating enclosure for use with an electronic oven of the character described, the heating enclosure being so dimensioned and the antenna being so placed therein as to provide excitation of a single predetermined electromagnetic field mode having fields varying in two dimensions and providing a uniform heating pattern over less than a wavelength of the magnetron microwave energy.

There has also been provided a vertically adjustable shelf for adjusting the position of the food load in the heating cavity to facilitate placement of the food in the appropriate region of the electromagnetic field pattern.

There has also been provided an improved disc type probe antenna which permits excitation of the preferred electromagnetic field modes with an antenna of substantially reduced dimensions.

There has also been provided a number of different forms of both the single mode and complementary mode embodiments of the heating enclosures of the present invention.

While there have been described what are at present considered to be the preferred embodiments of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. Electronic heating apparatus comprising a metal enclosure including six rectangular walls arranged to form a rectangular parallelepiped defining a heating cavity for receiving therein a body to be heated, and source means for transmitting microwave energy of a predetermined wavelength through one of said walls into said heating cavity for exciting therein a predetermined electromagnetic field mode, said one wall having an electrical dimension D such that wherein n is any integer and A is said predetennined wavelength, said predetermined electromagnetic field mode having the fields thereof constant in directions normal to said one wall with power flow being parallel to the direction of the dimension D, whereby there is established in said heating cavity an electromagnetic field pattern providing uniform heating of an associated y- 2. The electronic heating apparatus set forth in claim 1, wherein X is approximately equal to l2.9 inches.

3. The electronic heating apparatus set forth in claim 1, wherein said predetermined mode is a transverse electric mode.

4. The electronic heating apparatus set forth in claim 1, wherein said one wall is square with D )t/ V2, said source means including an antenna disposed centrally of said one wall and projecting into said heating cavity.

5. The electronic heating apparatus set forth in claim 1, wherein said one wall has an electrical width D and length E such that D= l.l hand said source means including an antenna projecting through said one wall into said heating cavity at a location spaced a distance D/4 from one of the edges of said one wall having the dimension E and spaced a distance E/2 from one of the edges of said one wall having the dimension D.

6. The electronic heating apparatus set forth in claim 1, wherein said one wall has an electrical width D and length such that vD= 1.1 Aand said source means including an antenna projecting through said one wall into said heating cavity at a location spaced a distance D/4 from one of the edges of said one wall having the dimension E and spaced a distance E/4 from one of theedges of said one wall having the dimension D. e v e 7. The electronic heating apparatus setforth in claim 1, and further including a shelf including a plurality of parallel spaced apart metal rods interconnected at the ends thereof and disposed in said heating cavity substantially perpendicular to the electric field for supporting the associated body to be heated, said shelf being selectively mountable at a plurality of different positions in said heating cavity for varying the position of the associated bodywith respect to the electromagnetic field pattern therein.

8. The electronic heating apparatus set forth in claim 1, wherein said source means includes an antenna projecting through said one wall into the heating cavity,

said antenna including a short cylindrical member disposed substantiallyperpendicular to said one wall and each side, said source means including an antenna disposed along said rear wall at a location substantially midway between said side walls and spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity.

14. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is A,

' and said heating cavity has a width of \/2 A, and a 'a circular disc connected to said cylindrical member at cavity for exciting therein two predetermined complementary electromagnetic field modes, at least one of said predetermined electromagnetic field modes having the fields thereof constantin a direction parallel to one of the dimensions of said; heating cavity, each of said predetermined electromagneticfield modes havingthe fields thereof varying in the direction in which the fields'of the other mode are constant, whereby there is established in said heating cavity an undistorted electromagnetic field pattern providing uniform heating of an associated body.

10. The electronic heating apparatus set forth in claim 9, wherein said-predetermined complementary modes are transverse electric modes.

ll."lhe electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength'is approximately equal to 12.9 inches.

12. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is A, and said heating cavity has a width of 1.2 A and a height of 0.633 A and a depth of 1.1 A, said source means being disposed along said rear wall at a location substantially midway between said side walls and spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity.

' l3. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is'A, and said heating cavity is a cube measuring A/ on ,height of A/ V2 and a depth of V2 A, said source means including an antenna disposed along said rear wall at a location spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity and spaced from one of said side walls a distance approximately equal to onefourth of the width of said heating cavity.

15. The electronic heating apparatusset forth in claim'9, wherein said predetermined wavelength is A, and said heating cavity is a cube measuring V2 A on each side, said source means including an antenna disposed along said rear wall at a location spaced from said top wall a distance approximately equal to onefourth of the height of said heating cavity and spaced from one of said side walls a'distance approximately equal t one-fourth of the width of said heating cavity.

16. The electronic heating apparatus set forth iii claim 9, wherein said predetermined wavelength is A, and said heating cavity has a width of 3 A/2 and a height of A/ V2 and adepth of 2 A, said source means including an antenna disposed along said rear wall at a location substantially midway between said side walls spaced from said top wall a distance approximately'equal to one-fourth of the height of said heating cavity.

17. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is A,

and said heating cavity has a width of 1.61 A and a height of 0.767 A anda depth of 1.23 A, said source means including an antenna disposed along said rear wall ata location substantially midway between said side walls and spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity.

proximately equal to one-fourth of the height of said heating cavity.

19. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is substantially equal to 12.9 inches.

20. Electronic heating apparatus comprising a'metal .enclosure in the form of a rectangular parallelepiped including a top wall, a bottom wall, a front wall, a rear wall and two opposed side walls defining a heating cavity for receiving therein a body to be heated, source electromagnetic field modes, at least one of said predetermined electromagnetic field modes having the fields thereof constant in a direction parallel to one of the dian associated body.

21. The electronic heating apparatus set forth in claim 20, wherein said source means includes an antenna disposed along said rear wall at a location substantially midway between said side walls and spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity, said tuning means being disposed centrally of said top wall.

q; 'STATESVPATEINTOFFICE l It is 'cert'ifiediYthajt eigror appears in the above-identified patent ahd that said Letters Pg tentkare hereby corrected a s shwn below z.

col'fimh, line 3 ""TT ismz zT shou d 1 0 2:z"']

pl'ced in different im pedarg c e.regions rof,thei F 15, line 4 filo de be o di 'Attstz (3mm, f I

' c. MARSHALL DANN Y F I i 1 3 Commissioner of Patents? i Att qsting- 'Officef and Trademarks ORM Poq so (10-69) U usc oMM-pc wave-pea I GOVERNMENT PRINT NG OFFICE I... O -3-3 l4.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,855,440 Dated Jan. 17 1975 I t r( James.E. Staats and Louis H. Fitzmayer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title page, References "Statts et al" should be -'Staats et al--;

Column 1, line 46, "electromagnet" should be --electroma gnetic--;

Column 3, line 16, "eating" should be --heating-;

Column 7, line 51, "pa tterh" should be --pattern--;

Column 8, line 30, "T TE h uld b "TE Column 10, line 59, after "impedance" insert "foods may be placed in different impedance regions of the Column 13, line 29, "cpncentric" should be --concentric--;

Column 13, line 47, "mode" should be --modes.-; and

Column 15, line 4, "mode" should be --modes--.

Signed and sealed this 4th day of March 1975.

Attest: C. MARSHALL DANN RUTH C. MASON 4 Commissioner of Patents Attesting- Officer I and Trademarks ORM PC4050 (10-69) USCOMM-DC scan-Pas U.S. GOVERNMENT PRINTING OFFICE l9" 0"35633. 

1. Electronic heating apparatus comprising a metal enclosure including six rectangular walls arranged to form a rectangular parallelepiped defining a heating cavity for receiving therein a body to be heated, and source means for transmitting microwave energy of a predetermined wavelength through one of said walls into said heating cavity for exciting therein a predetermined electromagnetic field mode, said one wall having an electrical dimension D such that D > OR = 1.1 n lambda /2 wherein n is any integer and lambda is said predetermined wavelength, said predetermined electromagnetic field mode having the fields thereof constant in directions normal to said one wall with power flow being parallel to the direction of the dimension D, whereby there is established in said heating cavity an electromagnetic field pattern providing uniform heating of an associated body.
 2. The electronic heating apparatus set forth in claim 1, wherein lambda is approximately equal to 12.9 inches.
 3. The electronic heating apparatus set forth in claim 1, wherein said predetermined mode is a transverse electric mode.
 4. The electronic heating apparatus set forth in claim 1, wherein said one wall is square with D lambda / Square Root 2, said source means including an antenna disposed centrally of said one wall and projecting into said heating cavity.
 5. The electronic heating apparatus set forth in claim 1, wherein said one wall has an electrical width D and length E such that D 1.1 lambda and E 1.2 lambda , said source means including an antenna projecting through said one wall into said heating cavity at a location spaced a distance D/4 from one of the edges of said one wall having the dimension E and spaced a distance E/2 from one of the edges of said one wall having the dimension D.
 6. The electronic heating apparatus set forth in claim 1, wherein said one wall has an electrical width D and length E such that D 1.1 lambda and E 1.2 lambda , said source means including an antenna projecting through said one wall into said heating cavity at a location spaced a distance D/4 from one of the edges of said one wall having the dimension E and spaced a distance E/4 from one of the edges of said one wall having the dimension D.
 7. The electronic heating apparatus set forth in claim 1, and further including a shelf including a plurality of parallel spaced apart metal rods interconnected at the ends thereof and disposed in said heating cavity substantially perpendicular to the electric field for supporting the associated body to be heated, said shelf being selectively mountable at a plurality of different positions in said heating cavity for varying the position of the associated body with respect to the electromagnetic field pattern therein.
 8. The electronic heating apparatus set forth in claim 1, wherein said source means includes an antenna projecting through said one wall into the heating cavity, said antenna including a short cylindrical member disposed substantially perpendicular to said one wall and a circular disc connected to said cylindrical member at the inner end thereof cpncentric therewith and normal to the longitudinal axis thereof.
 9. Electronic heating appAratus comprising a metal enclosure in the form of a rectangular parallelepiped including a top wall, a bottom wall, a front wall, a rear wall and two opposed side walls defining a heating cavity for receiving therein a body to be heated, and source means electrically coupled to said heating cavity at a location along said rear wall in the upper half thereof, said source means transmitting microwave energy of a predetermined wavelength into said heating cavity for exciting therein two predetermined complementary electromagnetic field modes, at least one of said predetermined electromagnetic field modes having the fields thereof constant in a direction parallel to one of the dimensions of said heating cavity, each of said predetermined electromagnetic field modes having the fields thereof varying in the direction in which the fields of the other mode are constant, whereby there is established in said heating cavity an undistorted electromagnetic field pattern providing uniform heating of an associated body.
 10. The electronic heating apparatus set forth in claim 9, wherein said predetermined complementary modes are transverse electric modes.
 11. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is approximately equal to 12.9 inches.
 12. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is lambda , and said heating cavity has a width of 1.2 lambda and a height of 0.633 lambda and a depth of 1.1 lambda , said source means being disposed along said rear wall at a location substantially midway between said side walls and spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity.
 13. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is lambda , and said heating cavity is a cube measuring lambda / Square Root 2 on each side, said source means including an antenna disposed along said rear wall at a location substantially midway between said side walls and spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity.
 14. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is lambda , and said heating cavity has a width of Square Root 2 lambda , and a height of lambda / Square Root 2 and a depth of Square Root 2 lambda , said source means including an antenna disposed along said rear wall at a location spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity and spaced from one of said side walls a distance approximately equal to one-fourth of the width of said heating cavity.
 15. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is lambda , and said heating cavity is a cube measuring Square Root 2 lambda on each side, said source means including an antenna disposed along said rear wall at a location spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity and spaced from one of said side walls a distance approximately equal to one-fourth of the width of said heating cavity.
 16. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is lambda , and said heating cavity has a width of 3 Square Root 2 lambda /2 and a height of lambda / Square Root 2 and a depth of Square Root 2 lambda , said source means including an antenna disposed along said rear wall at a location substantially midway between said side walls spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity.
 17. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is lambda , and said heating cavity has a width of 1.61 lambda and a height of 0.767 lambda and a depth of 1.23 lambda , said source means including an antenna disposed along said rear wall at a location substantially midway between sAid side walls and spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity.
 18. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is lambda , and said heating cavity has a width of 1.12 lambda and a height of 0.645 lambda and a depth of 1.12 lambda , said source means including an antenna disposed along said rear wall at a location substantially midway between said side walls and spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity.
 19. The electronic heating apparatus set forth in claim 9, wherein said predetermined wavelength is substantially equal to 12.9 inches.
 20. Electronic heating apparatus comprising a metal enclosure in the form of a rectangular parallelepiped including a top wall, a bottom wall, a front wall, a rear wall and two opposed side walls defining a heating cavity for receiving therein a body to be heated, source means electrically coupled to said heating cavity at a location along said rear wall in the upper half thereof, said source means transmitting microwave energy of a predetermined wavelength into said heating cavity for exciting therein two predetermined complementary electromagnetic field modes, at least one of said predetermined electromagnetic field modes having the fields thereof constant in a direction parallel to one of the dimensions of said heating cavity, each of said predetermined electromagnetic field modes having the fields thereof varying in the direction in which the fields of the other mode are constant, and tuning means disposed in said heating cavity and coupled to said top wall for coupling electromagnetic field modes other than said predetermined complementary modes and tuning said other modes off resonance, whereby there is established in said heating cavity an undistorted electromagnetic field pattern providing uniform heating of an associated body.
 21. The electronic heating apparatus set forth in claim 20, wherein said source means includes an antenna disposed along said rear wall at a location substantially midway between said side walls and spaced from said top wall a distance approximately equal to one-fourth of the height of said heating cavity, said tuning means being disposed centrally of said top wall. 